Condition monitoring techniques are regularly based on the measurement and subsequent analysis of vibration signals measured using casing mounted vibration transducers, such as accelerometers. The main problems associated with using casing mounted vibration transducers relate to their mounting as the vibrations measured are dependent on the transmission path from the source of the vibration to the transducer. In some situations, subtle details in the vibration signal may be attenuated by the transmission path leading to missed indicators of diminished machine condition. These transmission path effects also mean that casing mounted vibration transducers are typically permanently fixed to the structure, as small changes in transducer position can result in different vibration signals being recorded. When the equipment is located in a hostile environment the performance of these transducers may degrade over time. Casing mounted vibration transducers are also particularly sensitive to environmental noise. Although casing mounted vibration transducers do not typically impede the normal functioning of a piece of equipment, in many cases special arrangements are required to mount them on a piece of equipment. For example, many vibration transducers are required to be mounted on flat surfaces, close to the source of the vibration. Furthermore, these transducers are typically unidirectional, and such multiple transducers are necessary to obtain enough information to make a confident decision regarding the condition of a piece of equipment.
Electric motors and electric generators, or, more generally, electric rotating machines regularly form key parts of electromechanical systems. In recent years, the analysis of currents which may be measured from the power cables connecting the electrical rotating machines to the power source has been shown as a successful method for monitoring the condition of electromechanical systems. It has been shown that the currents that are induced in an electrical rotating machine change with operating conditions, often resulting in amplitude and phase modulations of large alternating current (AC) power supply currents. Changes in operating conditions related to defects such as broken rotor bars or rotor eccentricity may be related to the amplitude and frequency of modulations of the power supply currents. Motor current signature analysis (MCSA) involves analyzing measured current signals in the frequency domain, in order to diagnose and trend progressing defects. MCSA is attractive as it is relatively cheap to implement, and as the electric rotating machine forms part of the electromechanical system, the method may be thought of as nonintrusive. Primarily, MCSA has been used in the diagnosis of electric motor faults, though it has also been shown to react to changes in external loads, such as those caused by defects occurring in mechanical components such as bearings or gears.
Usually the frequency spectrum of a measured current signal is dominated by the AC power supply current. The electric motor and the attached mechanical system forming electromechanical system, cause modulations of the AC power supply current resulting in sidebands appearing in the frequency spectrum. Hence, the dominant AC power supply current may be thought of as a carrier wave. It is rare that the power supply is ideal; phase and amplitude modulations of the AC power supply current unrelated to the operating condition of the machine can occur. This is especially true in electric drive systems where control action and pulse-width modulation will result in the power supply current carrier wave being a non-stationary waveform. Similarly, it is often the case that the load acting on an electrical rotating machine may be transient. The non-stationary nature of the power supply current carrier wave results in the components owing to the power supply appearing to be distributed over a range of frequencies. This can increase the difficulty in assessing the operating condition of an electromechanical system.
From U.S. Pat. No. 5,461,329, by Linehan et al. there is a known method of dealing with the problem presented above by incorporating circuitry in the data acquisition system which changes the sampling rate of measured current signals in line with the changing frequency of the AC power supply current carrier wave. Thus a sampled data set containing only stationary carrier waves is achieved. By also only considering whole numbers of carrier waves, the method ensures that, once converted to the frequency domain, components owing to the discontinuities between the first and last sample are decreased. Thus the ease of identifying components in the frequency domain which may be matched to defects is increased.
From U.S. Pat. No. 6,993,439 B2 by Grosjean, there is a known method of converting a measured current waveform from the time domain to the spatial domain. The angular displacement of the rotor of an electrical rotating machine is identified using a characteristic in a measured current waveform. For example, the amplitude modulations resulting from commutator switching may be used to estimate the position of the rotor of a DC motor. The current waveform is then normalized to this angular displacement and analyzed in the frequency domain, thus allowing electrical rotating machines which do not rotate at constant angular velocity to be analyzed in the frequency domain.
The prior art described above, gives methods of decreasing the variability of the frequency spectrum of measured current signals. However, even when considering an electromechanical system operating at a constant angular velocity and supplied by an idealized power supply, resulting in a stationary power supply current carrier wave, the amplitudes of modulation sidebands caused by operating conditions of the electromechanical system are low relative to the power supply current carrier wave and its harmonics. This is particularly true in the consideration of faults occurring in the mechanical system to which the electric machine is attached. Furthermore, insufficient resolution of transducers used to measure current signals can lead to harmonic distortion. As a result it can be difficult to distinguish components owing to the operating condition of the electromechanical system from other more dominant components or from the noise signals owing from transducer noise, ghost noise from non-constant sources or from transient vibrations occurring in the environment of the electric rotating machine.